Borehole survey systems used for geological surveying and drilling of oil and gas wells generally map or plot the path of a borehole by determining borehole azimuth (directional heading relative to a reference coordinate such as north) and borehole inclination (relative to vertical) at various points along the borehole. For example, in one early type of prior art system, a tool or probe that contains one or more magnetometers for indicating azimuth or direction and one or more pendulums or accelerometers for indicating inclination is suspended by a cable and raised and lowered through the borehole. In such a system, the probe is stopped at several points along the borehole and the directional coordinates of the probe are determined. When sufficient measurements at discrete points along the borehole are made, a plot or map of the borehole can be determined relative to a desired coordinate system (e.g., a Cartesian coordinate system centered at the wellhead with the z-axis extending downwardly toward the center of the earth and the x and y axes extending in the direction of true north and true east, respectively). This early type of prior art system is subject to several disadvantages and drawbacks, including magnetometer errors that result from variations in the earth's magnetic field due to local mineral formations, the borehole pipe or casing, or magnetic storms. Further, these early systems resulted in high survey costs because of the necessity to stop the probe at many positions along the borehole path.
One proposal for simplifying the survey operation and decreasing costs is disclosed in U.S. Pat. No. 4,362,054, which is directed to a selective filtering method for determining borehole heading while a probe containing magnetometers is moving. Such a system is subject to the previously mentioned magnetic interference. In addition, in such a system, aliasing errors are introduced because of the data sampling employed and further errors result because of noise induced by abrupt changes in probe velocity and because of errors that result from changes in probe acceleration as the probe negotiates a change in borehole direction.
Various considerations have brought about an ever increasing need for borehole surveying apparatus that is more precise and compact than the above-discussed type of prior art arrangements. For example, modern gas and oil drilling techniques have brought about smaller diameter boreholes and often require that wells be closely spaced. In addition, it is not unusual for a number of wells to be drilled toward different geological targets from a single wellhead or drilling platform. Further, depletion of relatively large deposits has made it necessary to drill deeper and to access smaller target formations. Even further, in the event of a deep, high-pressure blowout, precise knowledge of the borehole path is required so that a relief well can be drilled to intercept the blowout well at a deep, high-pressure formation.
One proposal for providing a small diameter probe for a borehole survey system involves the application of inertial navigation techniques that previously have been employed to navigate aircraft, spacecraft and both surface and subsurface naval vessels. Generally speaking, these inertial navigation techniques utilize an instrumentation package that includes a set of accelerometers for supplying signals that represent acceleration of the instrumentation package along the three axes of a Cartesian coordinate system and a set of gyroscopes for supplying signals representative of the angular rate at which the instrumentation package is rotating relative to that same Cartesian coordinate system. Two basic types of systems are possible: gimballed systems and strapdown systems. In gimballed systems, the gyroscopes and accelerometers are mounted on a fully gimballed platform which is maintained in a predetermined rotational orientation by gyro-controlled servo systems. In effect, this maintains the accelerometers in fixed relationship so that the accelerometers provide signals relative to a coordinate system that is substantially fixed in inertial space, e.g., a Cartesian coordinate system wherein the z-axis extends through the center of the earth and the x and y axes correspond to two compass directions. Successive integration of the acceleration signals twice with respect to time thus yields signals representing the velocity and position of the instrumentation package in inertial space (and, hence, the velocity and position of the aircraft, ship or probe of a borehole survey system).
Prior art gimballed systems generally have not been satisfactory because of the size of the required gyros. Further, such systems do not readily withstand the shock, vibration and temperature encountered in the survey of deep boreholes. In addition, gyro drift, precession, sensitivity to g-forces and other factors seriously affect system accuracy.
In strapdown inertial navigation systems, the gyros and accelerometers are fixed to and rotate with the instrumentation package and, hence, with the aircraft, naval vessel or borehole survey probe. In such a system, the accelerometers provide signals representative of the instrument package acceleration along a Cartesian coordinate system that is fixed relative to the instrumentation package and the gyro output signals are processed to transform the measured accelerations into a coordinate system that is fixed relative to the earth. Once transformed into the earth-referenced coordinate system, the acceleration signals are integrated in the same manner as in a gimballed navigation system to provide velocity and position information.
In many prior art systems that utilize strapdown techniques (or hybrid strapdown configurations in which the accelerometers are gimballed relative to the longitudinal axis of the probe), the probe must be frequently stopped to correct for velocity errors that are caused by instrument drift. Repeatedly stopping the probe during a survey is undesirable in that it substantially increases the time required for the survey operation and thus results in higher costs.
One technique for minimizing or eliminating the need to stop the probe is disclosed in U.S. Pat. No. 4,542,647, which describes a two-gyro strapdown inertial navigation system. In that system, the gyro information for the third axis of the probe coordinate system is synthesized from available accelerometer and gyro signals. The system also utilizes probe velocity, as determined by cable feed rate, to implement aiding of the navigation system.
Although the system described in U.S. Pat. No. 4,542,647 provides a relatively rugged system with improved survey speed and accuracy, some disadvantages are present. Firstly, synthesis of the gyro information for the third probe axis adds noise to the system signals. Although the noise is of acceptable level while the probe traverses slanted or inclined portions of the borehole, useful azimuth information can be lost while the probe (borehole) is at or near vertical. Thus, when the proposed system is used to survey a borehole having vertical sections, the probe must be stopped periodically for gyro compassing. If a large portion of the borehole is vertical, the survey speed and accuracy improvement that is otherwise available is partially lost.